Composite Components are components that can hold other components and be added as a single unit to multiple pages. This technique has several implementations in blog posts and marketplace modules and are sometimes called "Snippets" in some frameworks.

Sitecore Foundation has an implementation of this component that behaves like you would expect with a few other options that make it even more compelling.

Going to rendering parameters, you'll find three settings that allow you to configure the component to be rendered asyncronously. By doing so, you'll allow the page to be loaded and then the content from the component to be requested and then rendered client side. 

This would be great for taking slower performing components and just forcing them to be loaded asyncronously. You can configure whether to render the component content both server and client side if desired and can even provide a base url that will be used when loading the composite content. 

That last part can allow you to serve personalized content even if your page is delivered through a CDN. By configuring the base Url to a non cached url, you can load your page from the CDN and then have the componetn fetch content directly from sitecore, where the content in the component can be set to leverage AB Testing or personalization.

Here's a video overview of the component and it's Async Features.